The present invention relates to the formation of a filter element, and more particularly to a method of and an apparatus for forming a filter element by pressing heated molds against the peak and valley folds of a ring-shaped corrugated blank and then cooling the blank to shape the same to true circular concentric configuration.
Filter elements are manufactured by cutting off a sheet of filter element material to a sector-shaped blank, folding the blank into a corrugated structure having alternate peak and valley folds creased along arcuate lines, and forcibly bringing the opposite edges of the corrugated sectorial blank into abutment against each other to form a ring-shaped filter element.
The filter element material is usually composed of a stack of a dense layer, an intermediate layer, and a coarse layer which are bonded together. The dense layer is made of material fibers closely massed and intertwined together which are highly resistant to becoming loose under tension and not stretchable enough to absorb strains produced when the filter blank is folded. Therefore, when the blank is forcibly brought into the ring shape, the peak and valley folds tend to produce angular edges, rather than being trued into concentric relation. The filter element blank with such angular edges on the folds or creases presents difficulty in automatic production of filter elements. More specifically, a filter element blank may not smoothly and stably be set on molds when it is to be shaped by the molds while being impregnated with resin, and a filter element blank may be ruptured when the molds are pressed against each other.
One solution to the above problems is disclosed in Japanese Laid-Open Patent Publication No. 59-36515 published Feb. 28, 1984. According to the disclosed proposal, a jig has concentric ridges corresponding to the peak folds of a ring-shaped folded filter element blank. The ring-shaped blank which is placed on the jig is heated at about 120.degree. C. for about 30 minutes. The disclosed arrangement does not however lend itself to mass production of filter elements since it requires many jigs and heating devices, and a long period of time is required to heat filter element blanks.